1. Field of the Invention
The present invention relates to the supervision and control of battery power consumption by a mobile station in a wireless communications system, such as a cellular radio system and, more particularly, to the management of the operations of such a mobile station based on changes in its battery capacity.
2. Related Prior Art Systems
The prior art includes cellular radio systems which have been operating in the United States since the early 1980s, and providing telephone service to an ever growing subscriber base, presently estimated at over 20 million subscribers. Cellular telephone service operates much like the fixed, wireline telephone service in homes and offices, except that radio frequencies rather than telephone wires are used to connect telephone calls to and from the mobile subscribers. Each mobile subscriber is assigned a private (10 digit) directory telephone number and is billed based on the amount of "airtime" he or she spends talking on the cellular telephone each month. Many of the service features available to landline telephone users (e.g., call waiting, call forwarding, three-way calling, etc.) are also generally available to mobile subscribers.
The architecture for a typical cellular radio system is shown in FIG. 1. A geographical area (e.g., a metropolitan area) is divided into several smaller, contiguous radio coverage areas, called "cells," such as cells C1-C10. The cells C1-C10 are served by a corresponding group of fixed radio stations, called "base stations," B1-B10, each of which includes a plurality of RF channel units that operate on a subset of the RF channels assigned to the system, as well known in the art. For illustration purposes, the base stations B1-B10 are shown in FIG. 1 to be located at the center of the cells C1-C10, respectively, and are shown to be equipped with omni-directional antennas transmitting equally in all directions. However, the base stations B1-B10 may also be located near the periphery or otherwise away from the centers of the cells C1-C10, and may illuminate the cells C1-C10 with radio signals directionally (e.g., a base station may be equipped with three directional antennas each covering a 120 degrees sector).
The RF channels allocated to any given cell (or sector) may be reallocated to a distant cell in accordance with a frequency reuse plan as is well known in the art. In each cell (or sector), at least one RF channel is used to carry control or supervisory messages, and is called the "control" or "paging/access" channel. The other RF channels are used to carry voice conversations, and are called the "voice" or "speech" channels. The cellular telephone users (mobile subscribers) in the cells C1-C10 are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units, collectively referred to as "mobile stations," such as mobile stations M1-M5, each of which communicates with a nearby base station. Each of the mobile stations M1-M5 includes a controller (microprocessor) and a transmitter-receiver, as well known in the art.
With continuing reference to FIG. 1, the base stations B1-B10 are connected to and controlled by a mobile telephone switching office (MTSO) 20. The MTSO 20, in turn, is connected to a central office (not specifically shown in FIG. 1) in the landline (wireline) public switched telephone network (PSTN) 30, or to a similar facility such as an integrated system digital network (ISDN). The MTSO 20 switches calls between wireline and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, stores subscriber service profiles, and provides for the operation, maintenance and testing of the system.
Access to a cellular system by any of the mobile stations M1-M5 is controlled on the basis of a mobile identification number (MIN) and an electronic serial number (ESN) which are stored in the mobile station. The MIN is a digital representation of the 10-digit directory telephone number assigned to each mobile subscriber by the home system operator. The electronic serial number (ESN) is assigned by the manufacturer and permanently stored in the mobile station. The MIN/ESN pair is sent from the mobile station when originating a call and its validity is checked by the MTSO 20. If the MIN/ESN pair is determined to be invalid (e.g., if the ESN has been blacklisted because the mobile station was reported to be stolen), the system may deny access to the mobile station. The MIN is also sent from the system to the mobile station when alerting the mobile station of an incoming call.
When turned on (powered up), each of the mobile stations M1-M5 enters the idle state (standby mode) and tunes to and continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment). When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system to find the "best" control channel (in the United States, there are 21 "dedicated" control channels in each cellular system which means that the mobile station has to scan a maximum number of 21 RF channels). When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, the mobile station remains "in touch" with the system and may receive or initiate a telephone call through one of the base stations B1-B10 which is connected to the MTSO 20.
To detect incoming calls, the mobile station continuously monitors the current control channel to determine whether a page message addressed to it (i.e., containing its MIN) has been received. A page message will be sent to the mobile station, for example, when an ordinary (landline) subscriber calls the mobile subscriber. The call is directed from the PSTN 30 to the MTSO 20 where the dialed number is analyzed. If the dialed number is validated, the MTSO 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MTSO 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M5 which is present in that cell will compare the MIN in the page message received over the control channel with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station, which then forwards the page response to the MTSO 20. Upon receiving the page response, the MTSO 20 selects an available voice channel in the cell from which the page response was received (the MTSO 20 maintains an idle channel list for this purpose), and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel. A through-connection is established once the mobile station has tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call (e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station), the dialed number and MIN/ESN pair for the mobile station are sent over the control channel to the base station and forwarded to the MTSO 20, which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as described before. If the mobile station moves between cells while in the conversation state, the MTSO 20 will perform a "handoff" of the call from the old base station to the new base station. The MTSO 20 selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MTSO 20 via the new cell. The old voice channel in the old cell is marked idle in the MTSO 20 and may be used for another conversation. Furthermore, when travelling outside the system, the mobile station may be handed off to a cell in an adjacent system if there is a "roaming agreement" between the operators of the two systems.
In order to properly direct incoming calls to a mobile station which is moving around between different cells or systems, it is necessary to keep track of the location and activity of the mobile station. For this purpose, an autonomous registration process has been used in which the mobile station sends a registration message to the system upon entering a new system area or a new location area (i.e., a predefined group of cells in the system), or at predetermined intervals defined by the system operator. The system area and location area registration functions can be used to identify the current location of the mobile station so that it can be paged in its actual (or most likely) location rather than in all locations in the system. Each time the system receives a registration message from a mobile station in its area, it marks this mobile station as being active and present in its system area, or in the particular location area containing the cell of the base station which received the registration message, and then sends a registration confirmation message to this mobile station. The periodic registration function, on the other hand, is used to determine whether a mobile station is active (powered and within radio range) in a cellular system. Incoming calls to inactive mobile stations can be routed immediately to a recorded message (e.g., "The mobile customer you have called has turned off the mobile unit or travelled out of the service area.") without ever paging these mobile stations. This reduces the paging load and results in more efficient use of the limited control channel capacity.
The primary parameters that regulate the various mobile registration functions include the next registration (NXTREG) value which is stored in each mobile station and the system identification (SID), location area identification (LOCAID), registration identification (REGID) and registration increment (REGINCR) values which are broadcast by the system on the control channel of each cell. The SID is a digital number which uniquely identifies the serving cellular system. The LOCAID is a digital number which identifies a particular location area comprised of one or more cells in the system. The REGINCR defines the length of the periodic registration interval. The REGID is a 20-bit counter that is stepped by one unit in every REGID message transmitted to the mobile station. The NXTREG value indicates when periodic registration is due and is calculated internally in the mobile station by adding the current values of REGID and REGINCR. A mobile station will register with the serving system if either the SID or LOCAID received over the control channel is different from the corresponding value which it stored the last time it received a registration confirmation message (thus implying that the mobile station has travelled to a new system or location area, respectively), or if the REGID value received over the control channel is greater than or equal to the stored NXTREG (thus implying that a periodic registration is due). The mobile station updates the NXTREG value (with the sum of the current REGID and REGINCR values) upon the receipt of each registration confirmation message and, also, after every successful voice channel designation (i.e., call originations and receptions are treated like normal periodic registrations since by making or receiving a call a mobile station shows its activity and location).
The original cellular radio systems, as described above, used analog transmission methods, specifically frequency modulation (FM), and duplex (two-way) RF channels in accordance with the Advanced Mobile Phone Service (AMPS) standard. According to the AMPS standard, each control or voice channel between the base station and the mobile station uses a pair of separate frequencies consisting of a forward (downlink) frequency for transmission by the base station (reception by the mobile station) and a reverse (uplink) frequency for transmission by the mobile station (reception by the base station). The AMPS system, therefore, is a single-channel-per-carrier (SCPC) system allowing for only one voice circuit (telephone conversation) per RF channel. Different users are provided access to the same set of RF channels with each user being assigned a different RF channel (pair of frequencies) in a technique known as frequency division multiple access (FDMA). This original AMPS (analog) architecture forms the basis for an industry standard sponsored by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA), and known as EIA/TIA-553.
In the late 1980s, however, the cellular industry in the United States began migrating from analog to digital technology, motivated in large part by the need to address the steady growth in the subscriber population and the increasing demand on system capacity. It was recognized early on that the capacity improvements sought for the next generation cellular systems could be achieved by either "cell splitting" to provide more channels per subscribers in the specific areas where increased capacity is needed, or by the use of more advanced digital radio technology in those areas, or by a combination of both approaches. According to the first approach (cell splitting), by reducing the transmit power of the base station, the size of the corresponding cell (or cell radius) and, with it, the frequency reuse distance are reduced thereby resulting in more channels per geographic area (i.e., increased capacity). Additional benefits of a smaller cell include a longer "talk time" for the user since the mobile station will use substantially lower transmit power than in a larger cell and, consequently, its battery will not need to be recharged as often.
While cell splitting held the promise of improving both capacity and coverage for the growing mobile subscriber base, the actual capacity gains were limited by the use of the analog AMPS technology. It was commonly believed that the desired capacity gains, and indeed the effectiveness of the microcellular (cell splitting) concept in increasing capacity, can be maximized only by the use of digital technology. Thus, in an effort to go digital, the EIA/TIA developed a number of air interface standards which use digital voice encoding (analog-to-digital conversion and voice compression) and time division multiple access (TDMA) or code division multiple access (CDMA) techniques to multiply the number of voice circuits (conversations) per RF channel (i.e., to increase capacity). These standards include IS-54 (TDMA) and IS-95 (CDMA), both of which are "dual mode" standards in that they support the use of the original AMPS analog voice and control channels in addition to digital speech channels defined within the existing AMPS framework (so as to ease the transition from analog to digital and to allow the continued use of existing analog mobile stations). The dual-mode IS-54 standard, in particular, has become known as the digital AMPS (D-AMPS) standard. More recently, the EIA/TIA has developed a new specification for D-AMPS, which includes a digital control channel suitable for supporting public or private microcell operation, extended mobile station battery life, and enhanced end-user features. This new specification builds on the IS-54B standard (the current revision of IS-54), and it is known as IS-136. (All of the foregoing EIA/TIA standards are hereby incorporated herein by reference as may be necessary for a full understanding of these background developments. Copies of these standards may be obtained from the Electronics Industries Association, 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006).
According to IS-54B and as shown in FIG. 2, each RF channel is time division multiplexed (TDM) into a series of repeating time slots which are grouped into frames carrying from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital speech channel. Each frame on the RF channel comprises six equally sized time slots (1-6) and is 40 ms long (i.e, there are 25 frames per second). The speech coder for each digital traffic channel (DTCH) can operate at either full-rate or half-rate. A full-rate DTCH uses two equally spaced slots of the frame (i.e., slots 1 and 4, or slots 2 and 5, or slots 3 and 6). When operating at full-rate, the RF channel may be assigned to three users (A-C). Thus, for example, user A is assigned to slots 1 and 4, user B is assigned to slots 2 and 5, and user C is assigned to slots 3 and 6 of the frame as shown in FIG. 2. Each half-rate DTCH uses only one time slot of the frame. At half-rate, the RF channel may be assigned to six users (A-F) with each user being assigned to one of the six slots of the frame as also shown in FIG. 2. Thus, it can be seen that the DTCH as specified in the IS-54B standard allows for an increase in capacity of from three to six times that of the analog RF channel. At call set-up or handoff, a dual-mode mobile station will be assigned preferably to a digital traffic channel (DTCH) and, if none is available, it can be assigned to an analog voice channel (AVC). An analog-only mobile station, however, can only be assigned to an AVC.
The IS-136 standard specifies a digital control channel (DCCH) which is defined similarly to the digital traffic channel (DTCH) specified in IS-54B (i.e., on the same set of RF channels and with the same TDMA frame format and slot size). Referring back to FIG. 2, a half-rate DCCH would occupy one slot while a full-rate DCCH would occupy two slots out of the six slots in each 40 ms frame. The DCCH slots may then be mapped into different logical channels which are organized into a series of superframes. FIG. 3 shows the superframe structure of a full-rate DCCH according to IS-136 (in this example, the DCCH is defined over channel "A" in the TDMA frame). A superframe is defined in IS-136 as the collection of 32 consecutive time slots (640 ms) for a full-rate DCCH (16 slots for a half-rate DCCH). The logical channels specified in IS-136 include a broadcast control channel (BCCH) for carrying system-related information which is broadcast to all mobile stations, and a short message service, paging and access response channel (SPACH) for carrying information which is sent to specific mobile stations.
As shown in FIG. 3, the BCCH is divided into logical subchannels each of which is assigned an integer number of DCCH slots. The BCCH subchannels include a fast BCCH (F-BCCH), an extended BCCH (E-BCCH) and a point-to-multipoint short message service BCCH (S-BCCH). The F-BCCH is used to broadcast DCCH structure parameters and other parameters required for accessing the system (the first slot in a superframe is always assigned to the F-BCCH). The E-BCCH is used to broadcast information that is not as time-critical (for the operation of the mobile stations) as the information in the F-BCCH. The S-BCCH is used for the broadcast short message service (SMS) which can deliver alphanumeric messages of common interest to all mobile stations (e.g., traffic reports). The SPACH is also divided into logical subchannels each of which is assigned a given number of time slots on a fully dynamic basis (and, thus, these subchannels are not explicitly shown in FIG. 3). The SPACH subchannels include a point-to-point short message service channel (SMSCH), a paging channel (PCH) and an access response channel (ARCH). The SMSCH is used for carrying alphanumeric messages of interest to a specific mobile station (e.g., stock quotations). The PCH is used for carrying paging messages to different mobile stations (each mobile station is assigned to a predefined "paging frame class" which defines the periodicity with which it reads the PCH). The ARCH is used for responding to access requests from one of the mobile station (e.g., by delivering a channel assignment message to this mobile station).
An idle mobile station operating on the DCCH of FIG. 3 need only be "awake" (monitoring) during certain time slots (e.g., the F-BCCH or its assigned PCH slot) in each DCCH superframe and can enter "sleep mode" at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power. Sleep mode operation reduces battery drain in the mobile station during idle mode and, therefore, increases both "standby time" and "talk time" for the user.
With the increase in standby time for an IS-136 mobile station, however, there is an increased risk that the user will leave a battery-powered mobile station turned on and idling in this mode for such a long period of time that the battery will discharge to the point that the simple action of generating a ring to the user (e.g., in response to a page message) will further discharge the battery to a level at which the mobile station will automatically turn off. This risk is further exacerbated in those situations where the mobile station must remain powered on for a period of time in order to accomplish a specific task. For example, a mobile station with a severely discharged battery may be connected to an external battery charger for rapid charging. In this case, the mobile station will turn on, begin the process of rapidly charging the battery and then attempt to register with the serving system. To transmit the registration message, the power amplifier (PA) in the transmitter will be turned on. As a result of turning on the PA, however, the battery voltage level will fall below the reset threshold and the mobile station then will turn off, thus ending the needed rapid charging of the battery.
In light of these problems, a mechanism is needed for the orderly shutdown of the various operations of the mobile station as its battery becomes increasingly discharged. It is further desirable that a discharged battery does not force the mobile station to automatically shut down merely because of the receipt of a page message. It is also desirable to avoid the premature shutdown of certain mobile station operations (e.g., rapid charging) when the battery is severely discharged. These goals are met by the present invention.